AMMAN, July 21 (Reuters) - Military support from Iran and its Shi'ite ally Hezbollah has given Syrian President Bashar al-Assad new impetus in his fight against the insurgents intent on ousting him, but at a price.

Assad now risks losing much of his autonomy to Tehran and becoming a pawn in a wider sectarian war between Sunni Muslims and Shi'ites that may not end even if he is forced to step down, military experts and diplomats in the region say.

Having lost thousands of troops and militiamen from his Alawite sect as the war grinds through its third year, and anxious to preserve his elite loyalist units, Assad is now relying on Hezbollah from Lebanon and other Shi'ite militias allied with Iran to turn the tide of battle.

Alawite army units with their vast arsenal of artillery and missiles have been taking a back seat in combat, using these weapons supported by the air force to obliterate rebellious neighbourhoods and blow holes in rebel lines for Iranian-and Hezbollah-trained local militias.

In some cases men from Hezbollah, an Iranian-backed group that is one of Lebanon's most powerful military and political forces, have been doing the street fighting, according to rebel commanders and other opposition sources.

Under this new arrangement, Hezbollah and Iran have become directly involved in the command structures of Assad's forces, eroding his authority and the Alawite power base that has underpinned four decades of family rule by him and his father.

The Alawites, to which Assad belongs, are an offshoot of Islam that has controlled Syria since the 1960s.

Unlike the Shi'ites in Iraq, Iran and Lebanon, Syria's Alawites tend to be secular and lack the religious zeal that has helped motivate thousands of Shi'ite militia to come to Syria.

Security sources in the region estimate there are about 15,000 Shi'ite fighters from Lebanon and Iraq in Syria, and they have helped produce success on the battlefield, reversing gains made by rebels in two years of fighting.

When rebel fighters have held confined areas, such as the border town of Qusair, which was overrun by Hezbollah and Assad loyalists two months ago, they have put themselves at a serious disadvantage, the sources said.

Rebellious Sunni districts in Homs to the south are being hit hard and Damascus suburbs, a main concentration of the Arab- and Western-backed Free Syrian Army, are under siege as the war's death toll climbs above 90,000.

But Assad's newfound military advantage may prove short lived, despite the increasing pressure on the rebels, military experts and diplomats believe.

The fall of Qusair, and Hezbollah's triumphant rhetoric, spurred regional heavyweight Saudi Arabia into action. The kingdom, diplomats say, has assumed the main role in backing the opposition in coordination with the United States.

TRAINED MILITIAS

Signs of renewed support for the opposition are showing in the northern city of Aleppo, where a government counterattack backed by Hezbollah, which trained Shi'ite militia in the area, has stalled, according to the opposition.

Even if Assad can capture Homs, hold Damascus and overrun neighbourhoods that had fallen to rebels, such as Jobar, Barzeh and Qaboun, he would preside over a much reduced country.

Kurdish fighters are consolidating their hold on a de facto autonomous region in the grain- and oil-producing northeastern province of Hasakah that came to being after Assad's forces withdrew to concentrate on defending areas in the interior.

Hardline Islamist brigades are ruling much of two provinces east of Hasakah and they are strongly present in Aleppo. Assad is mainly left with Damascus and a corridor running through Homs to his Alawite heartland and army bases on the coast and to Hezbollah's strongholds in Lebanon.

Andrew Terrill, research professor of national security affairs at the U.S. Army War college, said the rebels will "hang on" because Assad has lost too much of the country.

"Winning battles is very different than winning wars because people who are under assault are going to recoup at some point. The rebels remain armed and remain able to strike at him," Terrill told Reuters.

"Assad may be able to win in the sense that he may stay in power and he is not overthrown directly, but I cannot imagine him pacifying the country because I just think there are too many rebels and too much resistance," he said.

Terrill said new weapons expected from Saudi Arabia are bound to redress the balance of power as well as promised U.S. arms. Salim Idriss, head of the Free Syrian Army's command, is due to visit the United States this week to press for speedy U.S. arms shipments.

Iran meanwhile, continues to supply Assad with military assistance and financing estimated at $500 million a month, according to opposition sources.

"The Iranians and Hezbollah go in and train people and if they can whip these militias into shape then Assad could increasingly rely on them and spare his crack troops," Terrill said.

Hezbollah has openly acknowledged its involvement in Syria, but Assad and Iran have not commented.

PRAETORIAN GUARD

Faced with losing large areas of Syria to mainly Sunni rebel fighters, Assad has adjusted tactics in the last few months to preserve his mostly Alawite Praetorian guard units -- the Republican Guards, the Fourth Division and the Special Forces -- and started relying on Hezbollah, especially to capture the central region of Homs, the sources said

Mohammad Mroueh, a member of the Syrian National Council, said Hezbollah and Iran have been training the militias Assad is using for street fighting in Homs and have established, together with Iranian officials, operations rooms in the city.

"When there is an area where the army and the militia encounter stiff resistance, they're calling Hezbollah to do the fighting," said Mroueh.

Abu Imad Abdallah, a rebel commander in southern Damascus, said Hezbollah fighters and Iraqi Shi'ite militia were key to capturing two areas on the south-eastern approaches to the capital -- Bahdaliyeh and Hay al Shamalneh -- in recent weeks.

"They went in after saturation bombing by the regime. They are disciplined and well trained and are fighting as religious zealots believing in a cause. If it was the army we would not be worried," he said.

But veteran opposition activist Fawaz Tello said that using Hezbollah was a sign of Assad's weakness, pointing to his inability to rely on Sunnis who form the bulk of the army.

"Remember that Assad started this conflict with about a million men under arms between conscripts and the army and the security apparatus. Now more and more he is relying on foreign troops and without them he will lose, especially if the rebels begin to receive advanced weapons," Tello said.

Assad was now becoming an Iranian proxy, Tello said, while Mamoun Abu Nawar, a Jordanian military analyst, said the Syrian leader was forced to bow to the will of Tehran.

"He can no longer call a division head and tell him to bomb the hell out of this neighbourhood or that. His command has been eroded and the command structure is now multinational," Abu Nawar said.

A diplomat in the region put it more bluntly: "Whether Assad stays or goes is becoming irrelevant. The conflict is now bigger than him, and it will continue without him. Iran is calling the shots."